Silver-iron batteries are well known in the art, and are taught by Brown, in U.S. Pat. No. 4,078,125, and Buzzelli, in U.S. Pat. No. 4,383,015. These patents teach the use of perforated silver sheet or expanded silver screen supports containing active silver material for positive plates, multiply-microporous separators, and either sulfurized iron oxide and/or iron oxide hydrate negative plates according to the teachings of Jackovitz et al., U.S. Pat. No. 4,356,101, or sintered metallic iron negative plates.
The silver-iron battery is now generally considered more stable than the silver-zinc battery. The silver-zinc battery has always presented major problems of internal electrical shorts due to zinc dendritic growth from the negative plates through the separator system. Both battery systems are quite expensive, and are usually restricted to applications where the energy density of the battery is critical to the total system mission. An example of such an application is the propulsion system power source for underseas vehicles.
In the silver-iron battery, the silver electrode has been the primary life limiting component and the electrode that only performed well at a low discharge rate. The silver electrode art is quite extensive, and recent improvements over the old silver sheet plates have been made.
Salauze, in U.S. Pat. No. 2,833,847, taught mixing silver powder of less than 300 mesh, i.e. less than about 45 microns diameter, and nickel grains together, depositing them on both sides of a perforated or wire grid nickel or nickel plated steel support, and sintering the whole in a reducing atmosphere at about 800.degree. C. This provided a reticulated nickel grain skeleton enclosing the silver grains. About 70 parts by weight of silver powder was used per 30 parts of nickel grains. This structure prevented silver agglomeration during electrochemical reaction, so that the plate remained porous to electrolyte and was able to retain its initial capacity when in service.
Solomon, in U.S. Pat. No. 3,294,590, recognized some disadvantages of silver electrodes charging at two voltage levels, the argentous oxide level and the argentic oxide level, and taught silver electrodes made from Ag.sub.2 O, argentous oxide, having a particle size below 2.5 microns diameter. Silver powder this small was capable of anodic oxidation to only the argentous level before onset of substantial oxygen evolution.
Holechek et al., in U.S. Pat. No. 3,332,801, taught embedding a metal grid, preferably silver or silver-coated malleable base metal, in a mass of finely divided monovalent and/or divalent silver oxide, which could also contain up to 25% of finely divided silver, all being preferably less than 325 mesh, i.e., about 45 microns. The found that use of a resilient grid, such as one made of nickel, deformed during high compression levels from 423 to 17,625 kg/cm.sup.2 (3 to 25 tons/in.sup.2), and when the loaded grid later returned to its original shape it could cause surface cracks or weak areas in the plate. They also found that use of over 50% silver powder caused the initial voltage to immediately decrease with continued high rate discharge.
Langguth et al., in U.S. Pat. No. 3,353,998, taught enclosing AgO and/or Ag.sub.2 O particles in an electrically conductive, porous sheath of sintered nickel particles, where the sheath pores were filled with nickel hydroxide. The edges of the sheath were sealed to prevent silver migration. The resulting plate operated at two distinct potentials, and had a charge-discharge cycle life of over 1,000 cycles. This provided a rather complicated structure and process of manufacture, however. In fact, none of these silver electrodes is manufactured to be compatible with iron electrodes for a silver-iron battery system.
In other areas, Langer et al., in U.S. Pat. Nos. 3,749,604 and 3,953,241, relating primarily to battery separators, taught positive plates for a zinc battery, where the positive plates could be made by sintering a pasted oxide, silver powder or silver powder polymer to a silver grid. They could also be made by impregnating carbonyl plaques with silver compounds. In another area, relating to button cells, DiPalma et al., in U.S. Pat. No. 4,292,383, taught cathode material containing silver (I) oxide and a metal additive, selected from silver or nickel, which was effective to cause uniformly distributed silver upon discharge instead of forming layers. Both the silver and the nickel were added as powders having a particle size below 10 micron diameter. Additionally, a second additive selected from mercuric oxide, cadmium oxide, cadmium hydroxide or manganese dioxide could be added, to provide upon discharge, a stepped decrease in cell voltage, so that one could determine impending cell exhaustion. The voltage step could be observed visually, as when a display would dim, or it could be measured electronically. This, however, introduces a greater number of components into the cathode material and it may be difficult to provide a homogeneous mixture.
In other button cell publications, Krebs, in U.S. Pat. No. 3,655,450, teaches utilizing divalent silver oxide as the principal active material with a covering of a layer of monovalent silver oxide as a secondary active material. Muramatsu et al., in Japanese Patent Kokai No. 53-68831 (Application No. 51-144154), appears to teach an active material comprising major amounts of AgO of about 2 microns to about 5 microns average diameter, and a mixture of from about 2 wt.% to about 10 wt.% of silver powders, one having a particle size of from 0.01 micron to 0.1 micron, and the other, a particle size of from 5 microns to 10 microns. This use of small particle sizes is designed to eliminate the high voltage portion of the discharge curve and to reduce the internal cell resistance of the button cell. Kondo et al., in Japanese Patent Kokai No. 53-6840 (Application No. 51-82349), appears to teach mixing 100 parts by weight AgO with 2 parts by weight of fine silver powder having a particle size range of from 0.02 micron to 1 micron diameter, as a non-sintered anode in a button cell. A separate collector plate of Fe, Ni, Co, Ti, W and their alloys is placed next to the anode.
It is an object of the invention to provide a high discharge rate silver electrode, particularly adapted to the silver-iron couple, which electrode will have a long life and will not limit the performance of the silver-iron system.